In-process sensors, such as on-line sensors, are used widely in the chemical, pharmaceutical, and food processing industries to measure one or more characteristics, such as composition, temperature, pressure, or pH of a process fluid. All of these sensors have a surface by which the sensor interacts with the process fluid in order to make the desired measurement. For example, spectroscopic instruments interact with process fluid through some type of optical window. Normal operation of such sensors typically requires that the surface of the sensor be absolutely free of contaminants, such as organic growth, solids, films or coatings, in order to take accurate measurements. For many processes this requirement is difficult to achieve. Therefore, various methods have been developed for cleaning the surfaces of such sensors.
Some methods for cleaning in-process sensors require removing the sensor from service, either by physical removal of the sensor from the process installation or by isolating (valving off) the sensor from the process. Both of these methods can be time consuming, especially if the sensor surface fouls quickly. These methods are potentially dangerous, for example, if the process involves toxic or otherwise hazardous chemicals. These methods may also harm the equipment. Moreover, the process itself, in addition to the process measurement, may be suspended until after cleaning has been completed.
An upgrade to isolation of an in-process sensor for cleaning purposes is available in some systems, wherein a cleaning fluid is directed at the sensor during operation. These systems are limited to those where the process is not detrimentally affected by addition of the cleaning fluid. One example is in waste water treatment where clean, pressurized water and/or air is directed at a sensor for cleaning purposes.
Mechanical methods have also been developed for cleaning of in-process sensors. Such methods involve use of wipers, brushes, and the like to physically scrape contaminants off the sensor. Disadvantages of these methods include limited use with viscous process streams, the necessity to suspend the process measurement, and difficulty in designing a mechanical cleaning device into the process equipment, especially for a process containing corrosive or otherwise hazardous streams.
Ultrasound has been applied to cleaning of in-process sensors. The use of ultrasound generates cavitation near the sensor to remove solids. However, ultrasound is limited to use with low solids and viscosity process streams, at pressures below 100 psig, certain temperatures, and streams with low specific gravity.
U.S. Pat. Nos. 4,307,741 and 4,385,936 disclose an apparatus and a process for cleaning a probe inserted into a sample process stream. The apparatus comprises a canister containing a cleaning agent, from which the cleaning agent is discharged, mixed with water and pumped through a nozzle with a jet spray end directed at the probe. The method is described as an improvement over cleaning methods that involve (1) removal of a probe from a process, (2) use of ultrasound to vibrate process fluid as it passes the probe and (3) use of brushes and/or wipers while the probe is in service. Nevertheless, the apparatus and method rely on introduction of a material (i.e., cleaning agent) foreign to the process stream.
U.S. Pat. No. 5,185,531 provides a cleaner for an in-line optical sensor comprising a blade. The blade mechanically wipes the surface of a sensor window. Measurements from the sensor are suspended until the cleaning operation is complete and data re-stabilizes.
German Patent Application DE 35 38 313 A1 discloses a device to clean sensors in bodies of water such as ditches around oil storage tanks, where the sensors are contaminated with animal or plant material. The device is essentially a hose with a nozzle from which a pressurized fluid is discharged so as to impact the external surface of the sensor. The pressurized fluid is water and/or air, which generates bubbles or current in the water, creating an oscillatory motion at the surface of the sensor to remove contaminants.
All of the aforementioned methods include numerous limitations and disadvantages. Therefore, it is desirable to have a cleaning system for in-process sensors, that allow for in situ cleaning of the sensor surface while the sensor is in operation, that is, measurement need not be suspended during cleaning. It would further be advantageous to clean the sensor without introducing foreign material, such as cleaning fluids into the process. The present invention meets these needs.